This invention relates to a resistor element and, more specifically, to a resistor element designed to detect the flow rate of a fluid.
As well known so far in the art, some resistor elements used for a thermal type of flow rate sensors adapted to detect the amount of intake air in automotive engines, etc. include an alumina pipe around which a thin film or wire of platinum is spirally wound and a lead wire electrically connected with this thin film or wire and connected to both ends of the alumina pipe. Such resistor elements are provided with a protecting layer on the thin film or wire of platinum so as to protect it. In general, the protecting layer is formed of glass whose surface is so smooth that heat exchange can take place efficiently between the resistor element and the flow of a fluid, e.g., air without disturbing the flow of the fluid.
However, problems with the conventional resistor elements are that even though the glass layer having a smooth surface is used as the protecting film, atmospheric soot, dust, oil, water, etc. are deposited onto the protecting film in the form of a layer, which then acts as a heat insulator, making heat exchange between the thin film or wire of platinum and the surrounding air stream worse. This will in turn result not only in deterioration of the output characteristics of the detection signal showing the flow rate of air but in a slow response of the detection signal as well.
Another type of flow rate sensors, e.g., a wire-heating type of flow rate sensor using a single wire of platinum as a heating wire, is known as well. In order to avoid dust deposition, dust or oil deposits are burned off by the heated wire.
However, the above heating technique cannot be applied to a resistor element having a glass protective layer, because the glass protective layer cannot stand up to high temperature. Nor can this technique be applied to a resistor element including a platinum film, because the platinum film breaks when exposed to temperatures so high as to burn off deposits. Hence, this heating technique cannot be applied to the present resistor element for flow rate sensors.
In order to solve these problems, this invention seeks to provide a resistor element which is so unlikely to adsorb atmospheric foreign matters such as dust that its responsiveness and output characteristics can be improved.